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_ t <br />DRAFT <br />®RAFT ~ ~ <br />XXIII. Subsidence (2.05.6(6) and 4.20) <br />Powderhorn Coal Company, Inc., has completed a survey of structures and <br />renewable resource lands existing within the permit area. The applicant <br />has delineated several structures overlying the proposed underground <br />workings. The Ute Water District, which serves approximately 45,000 people, <br />has a water treatment plant, several storage tanks, and a 24 inch treated <br />water distribution pipeline above the existing Roadside mine workings. <br />Several raw water collection pipelines also cross the proposed Cottonwood <br />mining area. Limited alluvial deposits, which represent minor surficial <br />aquifers, do exist within the flood plain areas of Cottonwood Creek and Rapid <br />Creek drainages. <br />The proposed five-year mine plan does not involve additional mining below <br />the Ute Water Conservancy District facilities or supply pipelines. Mining <br />will, however, occur beneath Rapid Creek and Cottonwood Creek within the <br />permit area. Extraction is also projected beneath the raw water collection <br />pipelines within the Cottonwood lease area. In the case of both the supply <br />and raw water pipeline facilities, the worst possible anticipated consequence <br />of mining would be the disruption of the pipeline. In both cases, the <br />applicant has demonstrated that the materials necessary for repair are <br />readily accessible and that the disruption would not result in cessation of <br />service to the water district's water users. The applicant proposes to <br />prevent material damage to both the raw water collection pipelines and the <br />channels and associated alluvial aquifers of both Rapid Creek and Cottonwood <br />Creek through the retention of stable pillars. <br />The application contains an extensive analysis of the stability of pillars <br />proposed to prevent subsidence beneath the Rapid and Cottonwood Creek <br />drainages. The application also contains predictions of maximum subsidence <br />expected to occur if pillar failure follows cessation of mining in the <br />potentially affected areas. Each of these analyses is prepared in accordance <br />with the state-of-the-art of subsidence prediction. <br />The demonstration of proposed pillar stability, including calculations of <br />pillar factors of safety using a conservative method developed by A.X. <br />Wilson, is included within this submittal. Pillars and main corridors which <br />have 80 feet by 80 feet dimension are proposed and demonstrated to exceed <br />factors of safety of 2.92 under maximum anticipated overburden Loadings. <br />The amended life-of-mine plan for the Cottonwood lease area indicates that <br />mains have been located to directly underlie significant portions of the <br />two potentially affected creeks and the associated alluvial aquifers and <br />raw water collection pipelines. In addition, several significant portions <br />of the Cottonwood Creek channel is underlain by proposed room and pillar <br />panel sections. In these areas, retreat mining is proposed to result in <br />relatively conservative slabbed pillars measuring 20 feet by 80 feet, which <br />calculations included within Appendix G demonstrate to have a safety factor <br />in excess of 1.34. <br />